Semiconductor elements having conducting paths in a vertical direction of the elements for the purpose of coping with high voltage and large current are generally called power semiconductor elements (for example, IGBTs (Insulated Gate Bipolar Transistors), MOSFETs (Metal Oxide Semiconductor Field Effect Transistors), bipolar transistors, and diodes). A power semiconductor device including a power semiconductor element mounted on a circuit board and packaged with a sealing resin is used in a wide variety of fields including industrial equipment, automobiles, and railways. With the recent increased performance of equipment provided with power semiconductor devices, there is an increasing demand for increasing performance of power semiconductor devices, such as increasing rated voltage and rated current, and increasing the use temperature range (higher temperatures, lower temperatures).
The package structures of power semiconductor devices are mainly case structures. In a power semiconductor device having a case structure, a power semiconductor element is mounted on a heatsink base plate with an insulating substrate interposed, and a case is bonded to the base plate. The power semiconductor element mounted in the inside of the power semiconductor device is connected to a main electrode. The power semiconductor element and the main electrode are connected using a bonding wire. For the purpose of preventing insulation failure during application of high voltage, an insulative gel-like filler, such as a silicone gel, is typically used as a sealing resin for power semiconductor devices.
In a conventional power semiconductor device, the semiconductor element in the case is sealed or filled with a silicone gel. The loss modulus of the silicone gel at 25° C. at shear frequency of 0.1 Hz is 1.0×103 to 1.0×105 dyne/cm2, and the complex modulus thereof is 1.0×106 dyne/cm2 or less (see PTD 1 below).